Quinn was found guilty by a Liverpool Crown Court jury last month of assaulting Mr Noble causing him actual bodily harm.

During his trial, the court heard on September 1 last year Quinn's 10-year-old son and eight-year-old daughter were playing outside Mr Noble's home.

They started misbehaving with water pistols and became offensive towards the painter and decorator and his wife, Geraldine Noble, by swearing and making abusive gestures at her.

The court heard another neighbour saw Quinn by his home in Satinwood Crescent, Melling, and suggested he bring his children under control but he claimed Mr Noble's children had sworn at him in the past and criticised Mrs Noble.

Quinn ran towards Mr Noble, who was standing with his hands by his side, and headbutted him.

Mr Green had told the jury: "Quinn did not need to go near this man if he had no intention of attacking him or being aggressive."

Quinn, who had been in the Merseyside force for 12 years but has not worked since 2002 after being injured in a number of police car accidents, had claimed he acted in self-defence.

Quinn, a father-of-three, spent five years in the Army before becoming a police officer.

Although he was a serving police officer, he had been on sick leave and has been living on incapacity benefit for the last two years.

He was dismissed from the force at a disciplinary hearing last week.

Michael Wolff, defending, said Quinn has a medical condition, which means he has constant pain in his back and hands.

His health problem started after he was involved in a crash while he was a passenger in a police car in June 1999 - a condition which was aggravated by two further crashes while on duty and a bicycle accident.

Mr Wolff said when his son, who has cerebral palsy, finished school next year, the family intends to move out of the area.

Jailing him for four months, the judge Mr Justice Henriques told Quinn: "You headbutted a gentle and decent man, whom you recognised was a non-combative and peaceable man. You have shown absolutely no remorse. You still assert you were defending yourself when you behaved as you did."